Torlon and PEEK Polymer Labyrinth Seals Choice?
When selecting the material for a labyrinth seal to be used in a reciprocating compressor, thermal properties can be a major concern. Since plastics behave differently from metals, there are certain thermal properties that you need to consider when using a thermoplastic labyrinth seal. In this article, we are going to look at four key thermal properties you need to investigate, focusing on two of the most common choices for polymer labyrinth seals: Torlon and PEEK.
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Key 1: Glass Transition Temperature, Tg
In thermoplastics, the glass transition temperature is the temperature at which the polymer begins to significantly soften. For T < Tg, polymers are rigid but for T > Tg their behavior experiences a significant change. You will begin to see rubbery behavior, their strength and modulus of elasticity will begin to drop, and their coefficient of linear thermal expansion increases. In labyrinth seals, you need to avoid operating temperatures near the Tg. For PEEK, the glass transition temperature is around 289°F and it melts at about 660°F. Torlon has a glass transition temperature around 527°F, which is significantly higher than PEEK.
Key 2: Continuous Use Temperature, CUT
The CUT is established based on UL tests. In a nutshell, it is defined as the temperature to which a polymer can be exposed to for 100,000 hours before losing ½ of its strength. This property ties in with thermal aging, which is manifested by a significant increase in the brittleness of the polymer. However, it does not impact the performance of an installed seal. It is only a major concern for high temperature (300 to 350°F) environments. Note that PEEK and Torlon both have a CUT of about 480°F.
Key 3: Heat Deflection Temperature, HDT
According to ASTM D-648, the HDT is the temperature are which a standard test specimen (usually 0.5 x 5 x 0.5) subject to three-point bending under a load of 264 psi at the outermost fiber deflects 0.01. It’s often known as the heat deflection temperature, softening temperature, or heat distortion temperature. What this test is primarily looking for is the temperature at which the flexural modulus is reduced to 100 ksi. This test is used to determine short term heat resistance. For bearing grade PEEK, the HDT is 383°F while it is 320°F for unfilled PEEK. Bearing grade Torlon has a HDT of 534°F.
Key 4: Coefficient of Linear Thermal Expansion, CLTE
The CLTE is used to predict how the dimensions of a part change with temperature. Smaller CLTEs are associated with more dimensionally stable parts. For polymers, the CLTE will increase with temperature. For closer tolerance applications, the CLTE must be accounted for. Torlon has a CLTE of 1.4 x 10-5 in./in./°F. Unfilled PEEK has a CLTE of 2.6 x 10-5 in./in./°F, while bearing grade PEEK is at 1.7 x 10-5 in./in./°F.
Torlon and PEEK are very popular material choices for labyrinth seals in reciprocating compressors. However, when selecting a polymer sealing material it is important to take into account the effect that elevated temperatures can have on the behavior of thermoplastics. Don’t forget to check the glass transition temperature, continuous use temperature, heat deflection temperature, and coefficient of linear thermal expansion.
Check out the PEEK properites comparison charts provided by Advanced EMC Technologies.